Transducer drive and underwater detector system

ABSTRACT

A NOVEL SYSTEM IS DESCRIBED FOR DRIVING A REACTIVE TRANSDUCER SUCH AS CRYSTAL IN A MANNER WHICH UTILIZES THE FULL CAPABILITY OF THE TRANSDUCER AND ALSO IMPROVES THE OPERATION OF THE SYSTEM RESPONDING TO OUTPUT SIGNALS FROM THE TRANSDUCER. AN OSCILLATOR OPERATING AT THE RESONANT FREQUENCY OF THE TRANSDUCER PROVIDES DRIVING ENERGY VIA A DRIVING CIRCUIT HAVING ESSENTIALLY NONREACTIVE CHARACTERISTICS AT THE OPERATING FREQUENCY, THE DRIVE CIRCUIT BEING DIRECTLY COUPLED TO THE TRANSDUCER FOR PROVIDING COHERENT PULSES OF OPERATING ENERGY THERETO AT THE RESONANT FREQUENCY OF THE TRANSDUCER. IN ONE EXAMPLE A PUSH-PULL TRANSISTOR OUTPUT STAGE IS DISCLOSED AS ISOLATING THE FREQUENCY DETERMINING COMPONENTS OF THE OSCILLATOR FROM THE TRANSDUCER. AN EXTREMELY LOW IMPEDANCE OUTPUT STAGE IS THUS UTILIZED AND SERVES TO DRIVE THE TRANSDUCER EXACTLY AS ITS INHERENT RESONANT FREQUENCY. THE IMPROVED TRANSDUCER DRIVE SYSTEM AND METHOD IS DISCLOSED AS AN IMPORTANT PART OF AN IMPROVED UNDERWATER DETECTION SYSTEM WHEREIN THE RETURN ECHO PICKED UP BY THE TRANSDUCER IS AMPLIFIED BY A HIGH Q, NARROW BAND RECEIVER WHICH IS SIMILARLY TUNED TO THE RESONANT FREQUENCY OF THE TRANSDUCER. CIRCUIT DIAGRAMS AS WELL AS SYSTEM BLOCK DIAGRAMS ARE PROVIDED.

TRANSDUCER DRIVE AND UNDERWATER DETECTOR SYSTEM Filed Aug. 12. 1968 ,4 frog/V545' States Unite 9 Claims ABSTRACT F THE DISCLOSURE A novel system is described for driving a reactive transducer such as a crystal in a manner which utilizes the full capability of the transducer and also improves the operation of the system responding to output signals from the transducer. An oscillator operating at the resonant frequency of the transducer provides driving energy via a driving circuit having essentially nonreactive characteristics at the operating frequency, the drive circuit being directly coupled to the transducer for providing coherent pulses of operating energy thereto at the resonant frequency of the transducer. In one example a push-pull transistor output stage is disclosed as isolating the frequency determining components of the oscillator from the transducer. An extremely low impedance output stage is thus utilized and serves to drive the transducer exactly at its inherent resonant frequency. The improved transducer drive system and method is disclosed as an important part of an improved underwater detection system wherein the return echo picked up by the transducer is amplified by a high Q, narrow band receiver which is similarly tuned to the resonant frequency of the transducer. Circuit diagrams as well as system block diagrams are provided.

This application relates to an improved transducer drive method and apparatus and also to an improved underwater detection system wherein a relatively low-power driving source coupled directly to a transducer in combination with a high Q, narrow band receiver provides greatly increased underwater detection capabilities. A brief discussion of available equipment and the shortcomings thereof will be helpful in appreciating the problems of the art and the advantages gained by the present invention.

In the presently available underwater detection equipment (typically referred to as depth sounders) the transducer element used for imparting a compressional interrogation signal to the water and for responding to reflected energy is a reactive device. Quartz crystal transducers and barium titanate transducers are widely used at the present time, although magnetostrictive transducers can be used. The crystal transducers such as those made from quartz as well as the barium titanate transducers have the appearance from an electrical circuit standpoint as being a series resonant circuit. The mechanical motion of the crystal is believed to be a factor contributing to the series resonant effect. In many of the transducer assemblies commercially available on the market a small driving transformer is included within the package containing the crystal portion of the transducer assembly. In each case the transducer assembly exhibits a definite resonant frequency characteristic. I have found that at the resonant frequency of the crystal the driving transformer typically incorporated as part of the assembly has little effect on the transducer assembly exhibiting the characteristics of a series resonant circuit.

In view of the reactive nature of such transducer assemblies much effort has been devoted to the design of the driving system for the transducer to obtain a maximum transfer of useful power to the transducer assembly. The

Patented Jan. 26, 1971 basic approach has been to use a tuned circuit in the output stage of the driving circuit, an inductor-capacitor tank circuit typically being used and tuned to match the transducer. Within this basic approach two design philosophies have evolved in the sounder business for driving the transducer assemblies. One is based on a pulse discharge technique wherein an extremely short and time-decaying transmit pulse is applied to the transducer. In such systems a very broad band receiver is utilized in order to obtain the desired response to the short pulse. However the requirement of a broad band receiver in the pulse discharge type system leads to problems in that as a practical matter the receiver must either be fairly low in sensitivity or it must be extremely well shielded and designed to avoid instability. Even then, it tends to be subject to noise and oscillations. The other basic design philosophy referred to as being used in presently available equipment is that wherein the driving oscillator as well as the receiver are tuned to the resonant frequency of the transducer, but without the transducer being connected. Since the output or drive stage for the transducer includes a tuned circuit having reactive components, it is found in actual practice with available equipment that when the transducer is connected serious loading occurs causing the frequency to shift away from the resonant frequency of the transducer. As a result such systems fail to utilize the transducer in its most efficient mode.

A careful analysis of the electrical conditions as well as the physical movement of the transducer face when an attempt is made to drive the transducer at its resonant frequency by a pulse from a resonant tank circuit of either the pulse discharge type or the CW type in which the tank forms the frequency determining elements of the oscillator (power type) will show that the mechanical motion of the crystal actually changes phase and, in fact, undergoes recurrent phase reversals. It has been observed that not only does the mechanical motion of the transducer undergo repeated phase reversals, but also the phase of the driving signal periodically changes. This is apparently due to the interaction of the voltages generated by the transducer with voltage and current flow associated with the reactive components in the transducer driving tank circuit. The frequency of this phase change appears to be controlled almost exclusively by the mechanical response or mechanical Q of the transducer. The result of this is that a pulse, consisting of the resonant frequency of the transducer lbut in which the phase is repeatedly reversed every few cycles, is applied to the water and is directed to the bottom. The characteristics of the pulse echo from the bottom remain unchanged and eventually reach the transducer surface. The transducer crystal, being a high Q device will not respond to the repeated phase reversal even though the frequency of the echo is the resonant frequency of the crystal. What small transducer response there may be to the pulse is further reduced by the rejection of the receiver to this type of phase reversed information. Hence, the overall system efficiency is very poor even though the mechanical energy is actually imparted to the water.

A study of the electrical and mechanical conditions surrounding systems in which there is a transmitter oscillator driving an output tank circuit which, in turn, drives the transducer reveals that, even though the oscillator can be tuned to the exact transducer frequency with a substitute resistive load replacing the transducer, the frequency will shift substantially when the transducer itself is connected. This can be understood when it is pointed out that the output of a tank circuit of this sort is at highest impedance at its resonant frequency, whereas the transducer (which appears electrically to be basically a series resonant circuit) is at lowest impedance at resonance. This, therefore, at resonance, becomes a high impedance circuit attempting to drive a low impedance device. A shift off resonance will greatly lower the impedance of the driving circuit and raise the impedance of the transducer. Under these conditions it is understandable that a shift will occur and to a point of balance, so to speak. The fault here lies in the fact that if the frequency shifts, so as to accommodate the ability of the drive circuitry, the transducer crystal will simply cease to pro duce maximum motion and, hence, the power will not be imparted to the water. While the receiver may be tuned to the actual drive frequency applied to the transducer, it is seen from the above that this does not in practice correspond to the transducer resonant frequency. The transducer thus fails to respond mechanically in the desired manner to the drive, nand also the eturn echo, being off-frequency, does not cause maximum motion of the transducer.

In summary, it has been found that a phase reversal of the transmit pulse occurs repeatedly when the transducer is driven directly by, or is an integral part of the oscillator circuit. Whether the transmit pulse is of the pulse discharge, rapidly decaying type, or is of the C-W power oscillator type apparently makes little difference. The signal continually reverses phase every few cycles. A narrow band, high Q receiver, as well as the transducer itself, does not respond properly to the phase reversed signals. Also, as discussed above, the shift in operating frequency which occurs when an actual transducer is used with a resonant circuit tuned exactly to the transducer frequency results in ineicient utilization of the transducer.

It is therefore an object of the present invention to provide an improved drive system for a reactive transducer.

Another object of the present invention is to provide an improved underwater detection system having increased range capability as compared to present systems and wherein relatively low voltage signals are applied to the transducer.

Another object of the present invention is to provide an improved underwater detection system utilizing a low power coherent pulse drive circuit for driving a resonant transducer at its resonant frequency.

A further object of the present invention is to provide an underwater detection system wherein the transducer is operated at its resonant frequency by being driven by a pulse source which does not have a tuned reactive output circuit, with the receiver being tuned to the resonant frequency of the transducer.

In accordance with the teachings of the present invention a low impedance essentially nonreactive driving source is directly coupled to a reactive transducer with the driving source being operated at a frequency corresponding to the resonant frequency of the transducer. The driving source is operated in an essentially gated C-W manner of operation. Thus pulses of driving energy are applied to the transducer in essentially a continuous wave manner of operation of the duration of the application of driving energy to the transducer. The circuit connected to the transducer does not include a tuned circuit and therefore there is no interaction of the transducer with reactive drive components, and hence the problem of phase reversal is avoided. When the driving source is switched off by an appropriate switching network the transducer is then in a condition to respond to returned energy at exactly the frequency of the transducer. As a result the receiver coupled with the transducer via the switching network can be a narrow band, high Q receiver adapted to reject essentially all noise signals and respond only to the output signals from the transducer which are provided at the resonant frequency of the transducer. The transducer is therefore operated in its most eflicient manner by being driven at its resonant frequency, and simultaneously the receiver acts in its inherent capacity 4 of rejecting noise signals at frequencies other than the resonant frequency of the transducer.

The above as well as additional advantages and objects of the invention will be more clearly understood frorn the following description when read with reference to the accompanying drawings wherein,

FIG. 1 is a block diagram illustrating the inventive concepts as applied to an underwater detection system.

FIG. 2 is a schematic circuit diagram illustrating the details of one system incorporating the present invention for an underwater detection system.

Referring now to the drawings and in particular to FIG. 1 a regulated power supply 10 is shown as providing regulated alternating current to the timing drum 11 of the type typically used in underwater detection systems presently available on the market. The timing drum 11 provides timing signals 12 to the monostable Y multivibrator 13 which in turn gates a conventional oscillator 14 on and off in accordance with the timing signals from the timing drum 11 for the transmission of signals by a transducer into the medium being interrogated by the system. While the specific frequencies involved will Vary with the specific system, for purpose of illustrating the present invention the oscillator 14 is shown as operating at 50 kilocycles.

The oscillator 14 is coupled via a buffer unit 15 to a nonreactive low impedance drive source 16. By nonreactive it is meant that the drive circuit 16 has a power output circuit which does not include a reactive tuned circuit. Thus the drive source 16 isolates the oscillator from the transducer and can be referred to as providing coherent pulses of a fixed frequency. The output circuit 16A of the drive circuit 16 is coupled to a transmit-receive switch network 17 which can be of a type well known in the art. As explained hereinafter, a diode switching network is found to work well. The switch 17 serves to selectively couple the driver 16 to a reactive transducer element 18 shown as having an inherent resonant frequency of 50 kilocycles. It will be seen that the frequency of the reactive transducer 18 corresponds exactly t0 the frequency of the drive signals applied thereto.

During alternate times the switch network 17 couples a conventional receiver 20 to the transducer 18 so that the receiver 20 will respond to output signals from the transducer 18 generated due to return energy impinging upon the transducer. The receiver 20 is also shown as operating at 50 kilocycles and thus the oscillator, the transducer, and the receiver are all tuned to the same operating frequency. The absence of frequency determining components in the output circuit of the drive source 16 assures actual operation of the drive circuit at the resonant frequency of the transducer. Output signals from the receiver 20 are applied to a conventional display unit 21 such as an oscilloscope or a line tracing display unit common in the underwater detection art.

As noted above, it is of importance to observe in FIG. 1 that the reactive transducer unit is driven by a nonreactive driving circuit in a gated continuous-wave mode with the C-W energy being applied to the transducer at a frequency corresponding to the frequency at which the transducer is resonant. Also the receiver is tuned to the resonant frequency of the transducer. Thus the previously described physical phase reversals of the driving face of the transducer giving rise to phase reversed echoes to which the transducer and receiver will not respond are avoided. Furthermore the use of the low impedance drive source which effectively isolates the transducer from the oscillator prevents the above-described off frequency drive signals which result from interactions between a transducer and a tuned tank circuit. The associated receiving equipment can thus be a narrow band and high Q device, giving rise to high noise rejection capability. Thus by utilizing the nonreactive coherent pulse driving circuit, it is found that the problems of presently available equipment are greatly reduced.

Turning now to the specific circuit diagram of FIG. 2, the details of one system incorporating the teachings of the present invention will be described.

In the circuit diagram transistors Q13, 14, and 1S constitute a monostable multivibrator which is keyed on in response to rotation of the timing drum driven at an accurate speed by the power supply shown in FIG. l. In FIG. 2 the drum is shown as the signal key unit 30. The output of transistor Q effectively gates the 50 kc. oscillator provided by transistor Q1 having the tank circuit of capacitor C3 and inductor L1 in the collector circuit.

The 50 kc. signals of oscillator 14 are applied to transistor Q2 which acts as a buffer with the collector circuit being directly coupled to the bases of complementary symmetry transistors Q3 and Q4. Transistors Q3 and Q4 Vprovide a very low impedance driving source (about 3 ohms) for the transducer unit 18 shown as including a transformer 30 and a crystal 31. The diode switch arrangement provided by diodes D3 and D2 connects the transducer to the drive circuit via coupling capacitor C6 during the transmit mode. The arrangement is such that the transducer is actually driven exactly at its resonant frequency, while the reactive components C3 and L1 of the oscillator are isolated from any effects of the transducer which would normally tend to cause the phase reversal and frequency shift problems described above.

Diodes D4 and D5 are normally conductive when the transducer is being driven during the transmit mode. An input tank circuit for the receiver formed by inductor L3 and capacitor C7 looks like a high impedance to the transmit signal and therefore does not load it. This tank circuit also provides isolation between the transmitter and the front end of the receiver during transmission. When transistors Q3 and Q4 are not driving the transducer and diodes D4 and D5 are not conductive (during the receive mode) no tank circuit is formed and capacitors C7 and C8 act as coupling capacitors to couple the transducer to the receiver 20. Thus a novel variable impedance switch is provided by the switch circuit in FIGURE 2.

Capacitor C6 in the output circuit of the drive transistors Q3 and Q4 provides D.C. isolation for the drive circuit. Since in practice most capacitors have a certain leakage current, the inductor L2 is advantageously included in the diode switching network to hold the diodes D2 and D3 at D C. ground potential to further insure isolation of the receiver from the transmitter circuitry, thereby preventing pickup of unwanted noise, etc. The inductor L2 should be of a size to present a high impedance to the 50 kc. drive signal. Inductor L2 thus operates essentially as a choke, its value having been selected at 1 milliHenry in the 50 kc. system illustrated. The value of the coupling capacitor C6 is likewise not critical, a value of 20 microfarads having been found to work well in the system shown which operated with a forty-seven volt power supply across transistors Q2 and Q3-Q4. An eleven volt supply was used on the monostable circuit as Well as on the oscillator.

It has been found in practice that the techniques disclosed herein give rise to a very major improvement over 'any available equipment. One set of tests indicates that the underwater detection system provides a twenty-five fold increase in efficiency over systems presently available on the market. lIt is believed that the major improvement is directly attributable to the concept of driving the transducer unit with an essentially nonreactive coherent pulse driving circuit operating at the resonant frequency of the transducer unit, with the receiver coupled with the transducer also operating at the resonant frequency of the transducer unit.

While the invention has been disclosed by reference to the presently preferred embodiments it will of course be understood that such modifications and changes which become obvious to a person skilled in the art as a result of the teachings hereof are intended to be encompassed by the following claims.

What is claimed is:

l. An energy transmitting system comprising in combination: transducer means exhibiting the characteristics of a tuned circuit and having a resonant operating frequency; first circuit means providing output signals at said frequency; and second circuit means connecting said first circuit means to said transducer means, said second circuit means being substantially non-reactive at said frequency and operative to maintain the frequency and characteristic of the signals from said first circuit means substantially constant whether said transducer means is connected or disconnected as a load.

2. A system as defined in claim 1 wherein said transducer means includes a magnetostrictive element.

3. A system as defined in claim 1 wherein said transducer means includes a piezoelectric element.

4. The system of claim 1 wherein said first circuit means includes an oscillator operating at said resonant frequency and an isolating output drive circuit connected between said second circuit means and said oscillator, said drive circuit and said second circuit means operating to apply said operating energy to said transducer at said resonant frequency while isolating said oscillator from the reactive effects of said transducer means.

5. The system of claim 4 wherein said drive circuit includes a transistor having a base electrode and an emitter-collector circuit, said base electrode being coupled to said oscillator, and said second circuit means connects said emitter-collector circuit to said transducer means.

6. The system of claim 5 wherein said last named circuit means comprises a first diode and a capacitor connected in series circuit between said emitter-collector circuit and said transducer means, and a second diode having its anode connected to the Cathode of said first diode and its cathode connected to the anode of said first diode.

7. A system for transmitting signals at a first frequency and responding to echo signals at said first frequency to provide electric output signals at said first frequency comprising in combination: signal generating means providing electric signals only at said frequency; transducer means having a resonant frequency which is equal to said first frequency; signal coupling circuit means connected between said transducer means and said generating means for applying the signals of said generating means to said transducer means, said coupling circuit means being substantially non-reactive at said first frequency to thereby maintain the characteristics of the signals provided by said generating means substantially constant regardless of whether said transducer is connected to or disconnected from said generating means; and signal output circuit means connected to said transducer means and including isolation circuit means blocking the transfer of signals from said generating means directly to the output terminal of said output circuit means.

8. The system of claim 7 wherein said coupling circuit means comprises a capacitor connected in series circuit with a pair of oppositely poled diodes which are connected in parallel circuit.

9. The system of claim 8 including a high inductance member connected to said diodes and to a point of reference potential, said member having an inductance such that it acts as a choke to substantially block signals at said first frequency.

References Cited UNITED STATES PATENTS 2,400,796 5/ 1946 Watts et al 340-3 3,117,241 1/1964 Paynter et al. 3,214,753 10/1965 Dodge 340-384 RICHARD A. FARLEY, Primary Examiner U.S. Cl. X.R. S40-5, l5, 384 

